The influence of fraction size in radiotherapy on the late normal tissue reaction--II: Comparison of the effects of daily and twice-a-week fractionation on human skin.

A comparison of the normal tissue reactions to daily and twice-a-week fractionation over 4 to 6 weeks was performed on postoperatively irradiated parasternal fields in patients with breast cancer. The radiation used was electron beams at 12-13 MeV. We have previously reported that the overall treatment time was of importance for establishment of iso-effect relationships for the acute skin erythema. In this paper we present the degree of telangiectasia after 5 years. Generally, a more pronounced late reaction was found after twice-a-week fractionation than was predicted from the acute reactions. The findings imply that the cumulative radiation effect (CRE) formula has to be modified for both acute and late reactions in certain clinical situations. The main conclusion is that the iso-effect relationship varies significantly over the dose range used in clinical radiotherapy. In the dose range of 30 fractions between 1.6 and 2.1 Gy in 6 weeks, the slope of the curve for iso-effect dose versus number of fractions varies between 0.26 and 0.35, the beta/alpha-ratios between 0.22 and 0.37 Gy-1 and the RBE 2 Fractions/week/5 Fractions/week between 1.97 and 1.82.